A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to increase the density of devices on an integrated circuit, it is necessary to reduce the size and pitch of lines and other features. However, most lithography apparatus operate at or near their resolution limits. Various process techniques have been developed to enable features smaller than the minimum size imageable by a lithography apparatus to be created. For example, to etch lines narrower than the width of a line in resist, the exposed resist can be treated with an electron beam, causing the remaining resist to liquefy or plasticize and flow to partially close the exposed lines. Then, lines can be etched into the underlying substrate that are narrower than the lines printed in the resist. To expose lines at a pitch smaller than the minimum pitch imageable by the lithographic apparatus, double exposure techniques can be used—a first set of lines is imaged at double the desired pitch, then a second set again at double the desired pitch but with a positional offset equal to the desired pitch. U.S. Pat. No. 6,589,713 discloses a method using both of these techniques to print features of reduced width and pitch. These techniques are particularly useful for gate definition but may also be used on other feature types.
A problem that can occur with double exposure techniques is that the widths of the features printed in the two exposure steps differ. This is illustrated in FIG. 2 of the accompanying drawings which shows features A which have been imaged in a first step at a pitch of 2P and features B which have been imaged in a second step, again at a pitch of 2P but with a positional offset of P. The resulting pattern has a pitch of P. The A features have a width (critical dimension—CD) of W1 and the B features a width of W2. Due to imaging or process variations, for example focus and dose variations, W1 may not equal W2. Such variation in CD can prevent the devices so made from functioning correctly, reducing yield of the process.